In Placentalia, the fetus depends upon an organized vascular connection with its mother for survival and development. Yet, this connection was, until recently, obscure. Here, we summarize how two unrelated tissues, the primitive streak, or body axis, and extraembryonic visceral endoderm collaborate to create and organize the fetal‐placental arterial connection in the mouse gastrula. The primitive streak reaches into the extraembryonic space, where it marks the site of arterial union and creates a progenitor cell pool. Through contact with the streak, associated visceral endoderm undergoes an epithelial‐to‐mesenchymal transition, contributing extraembryonic mesoderm to the placental arterial vasculature, and to the allantois, or pre‐umbilical tissue. In addition, visceral endoderm bifurcates into the allantois where, with the primitive streak, it organizes the nascent umbilical artery and promotes allantoic elongation to the chorion, the site of fetal‐maternal exchange. Brachyury mediates streak extension and vascular patterning, while Hedgehog is involved in visceral endoderm's conversion to mesoderm. A unique CASPASE‐3‐positive cell separates streak‐ and non‐streak‐associated domains in visceral endoderm. Based on these new insights at the posterior embryonic‐extraembryonic interface, we conclude by asking whether so‐called primordial germ cells are truly antecedents to the germ line that segregate within the allantois, or whether they are placental progenitor cells. Incorporating these new working hypotheses into mutational analyses in which the placentae are affected will aid understanding a spectrum of disorders, including orphan diseases, which often include abnormalities of the umbilical cord, yolk sac, and hindgut, whose developmental relationship to each other has, until now, been poorly understood.

中文翻译：

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小鼠胎胎动脉连接：涉及原始条纹和内脏内胚层的范式，对人类发展具有影响。

在胎盘素中，胎儿依靠与母亲的有组织的血管连接才能生存和发育。然而，直到最近，这种联系才变得模糊。在这里，我们总结了两个不相关的组织，即原始条纹或体轴，以及胚外内脏内胚层如何协同作用，在小鼠胃中创建和组织胎儿-胎盘的动脉连接。原始条纹进入胚外空间，在此处标记动脉结合的部位并形成祖细胞池。通过与条纹接触，相关的内脏内胚层经历了上皮到间充质的转变，从而使胎外中胚层对胎盘动脉脉管系统，尿囊或脐前组织有所贡献。此外，内脏内胚层分叉到尿囊中，在尿囊中有原始条纹，它可以组织新生的脐动脉，并促进尿囊对绒毛膜的延伸，绒毛膜是胎儿与母体交换的部位。Brachyury介导条纹延伸和血管形成，而Hedgehog参与内脏内胚层向中胚层的转化。独特的CASPASE-3阳性细胞将内胚层中的条纹和非条纹相关区域分开。基于这些在后胚-胚外界面的新见解，我们通过询问所谓的原始生殖细胞是否确实是分离于尿囊内的生殖细胞的前体，或者它们是否是胎盘祖细胞来得出结论。将这些新的工作假设纳入影响胎盘的突变分析中，将有助于理解包括孤儿疾病在内的一系列疾病，